Symmetries in particle physics: from nuclear isospin to the quark model

TL;DR

This paper provides a pedagogical introduction to group representation theory in particle physics, illustrating how symmetries like isospin and SU(3) led to the quark model, emphasizing historical context and physical intuition.

Contribution

It offers a beginner-friendly, historically grounded approach to understanding Lie groups and symmetries in particle physics, culminating in the conceptual emergence of quarks.

Findings

Successful classification of mesons in SU(3) octet

Limitations of the Sakata model for baryons

Historical development of quark model from symmetry principles

Abstract

We present a concise pedagogic introduction to group representation theory motivated by the historical developments surrounding the advent of the Eightfold Way. Abstract definitions of groups and representations are avoided in favour of the physical intuition of symmetries of the nuclear interaction. The concept of nuclear isospin is used as a physical motivation to introduce SU(2) and discuss the main techniques of representation theory. The discovery of strange particles motivates extending the symmetry group to SU(3), at first in the context of the Sakata model. We highlight the successes in fitting mesons in the SU(3) octet, discuss the drawbacks of the Sakata model for baryonic classifications, and how the Eightfold Way finally led to the quark model. This approach has two major advantages: (i) the main concepts of the theory of Lie groups are introduced and discussed without ever losing touch with its applications in particle physics; (ii) it allows the beginner to study group theory while also becoming acquainted with the historical developments of particle physics that led to the concept of quarks. In particular, in this pedagogical path the quarks appear as yet another class of particles predicted from symmetry principles, rather than being introduced ad hoc for postulating an SU(3) symmetry, as usually done in the literature.